The background of the present invention will be described in a manner which refers to a list of references that is provided before the claims section of the present application.
Laser irradiation has been used for the cooling of matter such as, for example, dilute gases and solids. For gaseous matter, an extremely low temperature in diluted atomic gases can be obtained by Doppler cooling leading to the observation of Bose-Einstein condensates1,2. Recently, laser cooling of ultra-dense gas has been demonstrated by collisional redistribution of radiation3. For solid matter, laser cooling of solids (ie. optical refrigeration), which was proposed in the 1930s by Pringsheim4-6, achieves cooling by the annihilation of phonons and quanta of lattice vibrations during anti-Stokes luminescence.
Optical refrigeration exhibits advantages such as, for example, compactness, lack of vibration, cryogen-free, high reliability7 and so forth. Applications include, for example, an all-solid-state cryocooler8, and an athermal/self-cooling laser7. However, optical refrigeration requires special high purity materials with appropriately spaced energy levels and a high emission efficiency which explains why optical refrigeration research has been confined to the rare-earth doped glasses and direct band semiconductors.
Laser cooling of rare-earth doped glasses has been demonstrated in a rare earth-metal-fluoride glass (ZBLANP) doped with trivalent ytterbium ions by Epstein9. Since then, progress has been made particularly in ytterbium-doped glass with a recent record of ˜155 K cooling from an ambient temperature, surpassing a performance of a thermoelectric Peltier cooler10. However, cooling cycles typically stop around 100 K in rare-earth doped glasses because high energy levels in a ground state manifold become depopulated owing to Boltzmann statistics5.
Excitonic resonances dominate11-13 for laser cooling of semiconductors. The laser cooling of direct band gap semiconductors like GaAs14-17, is interesting as semiconductors exhibit efficient pump light absorption, low achievable cooling temperature and integrate-ability into electronic and photonic devices. Furthermore, it is possible for semiconductors to be cooled to below 10 K since carriers obey Fermi-Dirac statistics, which keeps the lower energy valence band still be populated5, 13, 18. Although several experimental15-17 and theoretical11-13,19,20 works (typically on GaAs quantum wells) have been discussed, no net-cooling has been achieved in semiconductors. This is because of high parasitic background absorption and poor luminescence extraction efficiency, even though anti-Stokes up-conversion can be readily achieved13-15, 17.